1. Field of the Invention
The present invention relates to an optical recording medium for an optical recording and/or reproducing apparatus equipped with an optical pickup of a laser beam, and, particularly, relates to an optical recording medium for a high density recording and reproducing by employing a recording medium having a variable transmittance corresponding to the change of temperature to cause an effective spot diameter of the laser beam to be minimized.
2. Description of the Related Art
Recently, a possibility of high density recording of optical discs has been examined and various kinds of optical discs have been proposed.
A recorded mark smaller than a spot of a laser beam can be recorded on the optical disc by controlling a power of the laser beam. Thus, it is considered that the recording density of the optical disc: has no limitation in principle. The spot diameter of the laser beam when using a lens system, however, has a minimum limitation determined by focusing characteristics of the lens system. Thus, the possibility of the high recording density of the optical disc depends on the minimization of the spot diameter of the laser beam.
Generally, a reproducible repetition wavelength (recording wavelength) derived from the recorded marks is given by a relation of .lambda./2NA, wherein ".lambda." designates a wavelength of a light to be used and "NA" a numerical aperture of a lens.
As seen from the relation of .lambda./2NA, in order to discriminate and reproduce a shorter length or interval of the recorded marks, it is effective to employ a light having a shorter wavelength and/or a lens having a larger numerical aperture. However, it is difficult technically to produce a short-wavelength semiconductor laser and is not easy to install such a lens having a larger numerical aperture in the optical disc apparatus.
There has been proposed a high density recording and reproducing method employing an optical disc provided with an optical masking layer made of a variable optical transmittance (referred to as a variable transmittance hereinafter) material of which optical transmittance increases at a higher temperature by being irradiated with the laser beam and decreases at a lower temperature by being cooled in the atmosphere after the laser beam has passed through the layer as shown in FIG. 1.
FIG. 1 is a graph showing a relation between an intensity of light (i.e., temperature) and a transmittance in a variable transmittance material in the prior art.
Generally, an optical intensity distribution (referred to as an intensity distribution) of the laser beam used in the information recording and reproducing is given by Gaussian distribution and a temperature distribution thereof is given by approximately the same as the intensity distribution thereof.
FIG. 2 is a plan view for explaining a masking effect of a optical masking layer (referred to as a masking layer).
Referring to FIG. 2, a laser beam spot "P" is composed of an inner spot "A" coaxially including a center thereof and an outer spot "B" circularly surrounding the inner spot "A". Naturally, the light intensity of the inner spot "A" is larger than that of the outer spot "B". When such a laser beam "P" as mentioned in the above is irradiated on the variable transmittance material, a first irradiated area with the inner spot "A" only allows the laser beam to pass through due to its higher transmittance, on the other hand, a second irradiated area with the outer spot "B" prevents the laser beam from passing through because of a lower temperature unlike the first irradiated area, thus, a masking effect occurs.
FIG. 3 is a graph showing a relation between an irradiated spot diameter and an effective spot diameter of the laser beam.
Referring to FIG. 3, a numeral 21 designates a light intensity distribution of the an irradiated laser beam "P" and 22 a light intensity distribution of the inner spot "A". A spot diameter "De" of the inner spot "A" (referred to as an effective spot diameter) passing through the first irradiated area of the variable transmittance material layer becomes substantially smaller than an irradiated spot diameter "D" of the laser beam "P". Therefore, it is possible to minimize cross-talks between neighboring tracks of a series of information pits (recorded marks) and between neighboring information pits by providing the variable transmittance material layer (referred to as a masking layer hereinafter) in an overlayed state on the optical disc because the first irradiated area with the inner spot "A" only allows the laser beam to pass through the masking layer and the second irradiated area with the outer spot "B" prevents the the laser beam from passing therethrough.
As one of requirements for the masking layer, the masking layer needs to have speedy changeability and recovery of the transmittance in response to the irradiation of the laser beam. Specifically, when the optical disc is rotated and a laser beam is irradiated thereon for reproducing, the transmittance of the first irradiated area on the masking layer has to be changed large enough to allow the laser beam to pass therethrough while the laser beam is being irradiated on the rotating optical disc, and the change of transmittance thereof has to be recovered before the first irradiated area returns to the position of the laser beam again due to a disc revolution.
As the variable transmittance material substantially satisfying such a requirement, a multicomponent type thermochromic material is well known and the optical recording medium utilizing the above material is disclosed in Japanese Patent Laid-Open 6-162564/1994. As the multicomponent type thermochromic material, there is disclosed, for example, in Japanese Patent Laid-Open Publication 50-75992/1975 a three-component type thermochromic material composed of an electron donative color compound (leuco dye) as a dye reductant, an electron acceptant developer (simply referred to as a developer) as a solid-state acid and a polarized compound.
In the above multicomponent type thermochromic material, coloring, decoloring and color-change occur by exchange of electrons between the electron donative color compound and the electron acceptant developer. Generally, it is considered, however, that neither the decoloring nor a thermochromic phenomenon (reversibility) occurs as long as employing only these two kinds of compounds even when the temperature changes, because the exchange of electrons always occurs between the electron donative color compound and the electron acceptant developer irrespective of a change of temperature.
When a plastic polarized compound is mixed to these two kinds of compounds, the decoloring occurs at a temperature higher than a predetermined temperature by being heated due to the exchange of electron corresponding to the thermal equilibrium, and the coloring and the color-change occur at the room temperature, i.e., the thermochromic phenomenon (reversibility) is presented.
The multicomponent type compound has such features that a light absorbing wavelength range can be selected by alterring the structure of the electron donative color compound, and the temperatures, i.e., threshold values, of the coloring, decoloring, and color-change can be optionally selected by alterring the structure of the polarized compound.
As other examples of the thermochromic material than the above three-component type thermochromic material mixed with the plastic polarized compound for the reversibility, there are ones of which reversibilities are controlled by thermal energy or ones of which coloring and decoloring are performed by a phase-transformation of the developer as seen in the thermochromic materials used in thermographic papers.
In the thermochromic material of which reversibility is controlled by thermal energy, an amphoteric developer having both properties of the acidic group and the basic group is employed. The coloring density of this material changes corresponding to the thermal energy supplied and shows a maximum value at a certain level of supplied thermal energy, and after that, it decreases when the thermal energy larger than the certain level is applied. Thus, the coloring thereof is performed by cooling it rapidly after having heated it at the maximum value, and the decoloring thereof is performed by cooling it slowly after having heated it again at the maximum value.
In the thermochromic material of which coloring and decoloring occur due to a phase-transformation of the developer, there is a developer having a long chain structure. In this case, the coloring thereof is performed by cooling rapidly after having heated it at a maximum value, and the decoloring thereof is performed by cooling it slowly after having heated it again at the maximum value as well as the above case.
In the above mentioned thermochromic material employing the amphoteric developer or the long chain structure developer, the developer has the function of the decoloring, thus, the transmittance of the thermochromic material can be reversely changed by employing only the two component material, i.e., a two-component type thermochromic material composed of the electron donative color compound and the electron acceptant developer without the plastic polarized material. Thus, it have an advantage that the coloring density thereof can be made larger.
However, the above three-component type thermochromic material has a drawback that its coloring density is smaller because it requires much quantity of the polarized material to be contained for generating the decoloring effect. Thus, the above three-component type thermochromic material needs a very thick masking layer to cause the coloring, the decoloring and the color-change sufficiently, i.e., to enhance the light absorption effect of the masking layer. The larger the thickness of the masking layer, the less transparent the masking layer becomes, which poses a problem of degrading the S/N of the reproduced signal waveform.
On the other hand, the above two-component type thermochromic material employing the amphoteric developer or the long chain structure developer has an advantage that the coloring density thereof can be made larger without employing the plastic polarized material. Thus, the masking layer can be made thinner. However, there are drawbacks that the reaction speed thereof is too slow, so that the change of the transmittance thereof does not occur within the laser irradiated area or the change of transmittance thereof do not recover before the irradiated area returns to be re-exposed to the laser beam again due to the disc rotation. Further, it needs heating both in coloring and decoloring. In addition, upon coloring and decoloring, it needs temperature controls including rapid cool down and gradual cool down.
As a countermeasure thereof, for instance, it may be considered to employ plural laser beam spots or an elliptic laser beam spot or an elongated laser spot in the direction of an array of recorded pits, which poses a problem that the apparatus becomes too complicated. Accordingly, the two-component type thermochromic material mentioned above or the long chin structure is not feasible as the masking layer.
In order to provide the above mentioned 3-component thermochromic material layer on the optical disc, it is advantageous to employ a spin-coat method suited for forming a thick layer in the aspect of productivity thereof. Generally, in the spin-coat method, a liquid coating material is dropped on a rotating disc surface, and the dropped material is spread over the surface due to the centrifugal force coating the surface uniformly along thereof including contours if any. Because of this principle, it is difficult to form the masking layer of an uniform thickness on the uneven optical disc having pits and lands by the spin-coat method. Specifically, the pits and guide grooves form recessed portions on a substrate of the optical disc and the substrate itself forms a flat portion (land) except for the pits and the guide grooves. The thickness of the masking layer formed on the recessed portions is different from the thickness thereof formed on the flat portion, thus, the uniform thickness of the masking layer can not be obtained by the spin-coat method. The uneven thickness of the masking layer causes differences of an optical path length, optical and thermal characteristics between the flat portion and the recessed portions, which poses impossibility of obtaining precise reproducing signals.
Especially, the sizes of the pits and the guide grooves formed in a high density recording optical disc are made smaller than those of the ordinary optical discs practically used. In that case, the pits and the guide grooves may be filled with the masking material by the spin-coat method, which will lead to impossibility of reproducing of information signals from the optical disc.